In many data communication systems data are transmitted by modulating digital data onto carriers. Such data transmission systems include single-carrier data communication systems such as QAM (Quadrature amplitude modulation) or PSK (Phase shift keying) and multi-carrier system using a plurality of subcarriers such as OFDM (Orthogonal frequency division multiplexing) or DMT (Discrete multitone modulation) to transmit data on multiple frequency bands.
Vector data transmission (sometimes referred to as vectoring or vectored data transmission) is a technique for coordinated transmission of data from a plurality of transmitters to a plurality of receivers via a plurality of transmission links (transmission channels) in order to improve the transmission. Vector transmission for example reduces the influence of cross-talk by providing compensation or precompensation of the cross-talk induced onto the signal during transmission over the link. Vectoring is sometimes also referred to as Spectrum Management Level 3.
One type of cross-talk appearing in such transmission systems is the so called FEXT (far end cross-talk). For example in Very High Speed Digital Subscriber Line (VDSL) systems, FEXT can possibly be a major source of performance degradation. Typically for such transmission systems, data are transmitted from a central unit such as a central office (CO) or other provider equipment to a plurality of receivers located in different locations, for example in customer premises (CPE), via a plurality of communication channels. FEXT resulting from signals on different channels (lines) transmitted in the same direction results in a reduced data throughput. Through vectoring, signals transmitted over the plurality of communication channels from the central office or received via the plurality of communication channels in the central office may be processed jointly in order to reduce such cross-talk. In this respect, the reduction of cross-talk by coordinated transmission of signals is sometimes referred to as cross-talk precompensation, whereas the reduction of cross-talk through joint processing of the received signals is sometimes referred to as cross-talk compensation or cross-talk cancellation. The communication channels which are processed jointly are sometimes referred to as vectored group. Thus, to reduce or eliminate the FEXT influence, FEXT compensation can be used for upstream communication and FEXT precompensation can be used for downstream communication.
For this kind of cross-talk reduction cross-talk coefficients which are parameter describing the cross-talk between the communication connections have to be determined and are thereafter stored in order to use these parameters for cross-talk compensation or cross-talk precompensation. Cross-talk coefficient can be learned for example during an initialization of the system or during a joining of a new line to the vector group. During operation (showtime) of the system, the cross-talk coefficients are tracked. Learning as well as tracking require both a determining of the cross-talk coefficients and an updating of the previously stored cross-talk coefficients.